The present invention relates to an injection molding system for serially molding articles and particularly to such a system having apparatus for serially removing the molded articles from the injection molds. More particularly, the invention relates to such an injection molding system which operates continuously, and further includes apparatus for serially removing the articles from the injection molds and apparatus for serially transporting the molded articles away from the injection molds. The invention has particular utility in the formation of parisons or preforms for the manufacture of plastic bottles.
Plastic bottles can be produced from a parison generally resembling a thick test tube which is injection molded to include a completely formed finish including, where desired, threads or other features to receive a bottle closure, and a neck ring to facilitate handling of the parison and bottle. In the so-called two-stage process, the parison is cooled generally to room temperature after molding, and can be stored for substantial periods of time before subsequent blow molding into a bottle. At the time of blow molding, the parison is fed into a machine where the parison is re-heated and blown with compressed air to form the finished bottle.
An advantage of the two-stage process is the substantial flexibility achieved by allowing the injection molding and blow molding operations to occur at different locations and times. Additionally, because the parisons are so much smaller in volume than the finished bottles they produce, the parisons are easier and more economical to handle and transport. The injection molding step is generally more time consuming than the blow molding step. To avoid any inefficient use of the blow mold, the number of injection molds and blow molds can be coordinated so that the production of parisons by the injection process matches the production capacity of the blow molding apparatus. This enables one to use both the injection and blow molding processes at maximum time efficiency by coordinating the number of each type of mold with the duty cycle required by each type of mold to achieve maximum volume production.
Some systems have been developed for simultaneously injection molding as many as ninety-six parisons. In typical simultaneous parison molding operations, a set of cores and a set of cavities are arranged to reciprocate horizontally between a "closed" position where molten resin is injected to form the parisons and an "open" position where the parisons are ejected from the injection molding machine. Production is maximized by shortening the duty cycle of the injection molding machine which results in the parisons being ejected at higher temperatures. However, at higher temperatures, the parisons are increasingly susceptible to plastic deformation which can contribute to significant bottle non-uniformity in any subsequent blowing operation. The plastic deformation is particularly acute if the parison is retained at high temperature in a horizontal attitude while supported only by one end of the parison, typically the finish or the support ring.
Another problem with the simultaneous injection molding of a large number of parisons or other articles in a single operation resides in the need for batch processing of a significant volume of resin prior to actual injection. As the volume of the batch increases, the opportunity for thermal degradation of the resin also increases due to increased residence time of the resin at elevated temperature. To eliminate the resin residence time problem while still molding a large number of articles such as parisons, it has been proposed to couple a single resin plasticizer to a plurality of mold units as in Stroup, et al., U.S. Pat. No. 4,470,796. Stroup incorporates multiple molding cavities in each mold and operates the plurality of mold units in relation to each other such that the plasticizer can be operated in a continuous rather than a batch mode. Stroup also uses a plurality of transfer devices situated around the periphery of the mold units to transfer the molded parisons to a plurality of rotary stretch blow molding machines. The use of the plurality of transfer devices represents a significant capital investment which might be avoidable if the mold units could all be serviced by a single transfer device for transferring the molded parisons to subsequent processing equipment.
Thus, there is a need for a continuous injection mold system that serially molds a plurality of articles and then serially transfers the articles to subsequent processing equipment such as a blow mold to ensure that each finished article has the same pressure-temperature history. There is also a need for a continuous injection mold system which maintains the molded articles in an upright orientation to minimize any distortion of the articles as they cool.